The present invention relates to a resin-coated metal sheet obtained by laminating a film of a polyester composition on a metal substrate. More specifically, the invention relates to a resin-coated metal sheet having excellent shock resistance (dent resistance), resistance against high temperature and humidity, corrosion resistance, adhesiveness and workability, and to a metal can and a can closure made by using the above resin-coated metal sheet having the above properties.
A side-seamless can has heretofore been produced by subjecting a metal blank such as an aluminum plate, a tin plate or a tin-free steel plate to at least one step of draw working between a drawing die and a punch to obtain a cup which includes a barrel without seam on the side surface thereof and a bottom integrally connected to the barrel without seam, and, as required, by subjecting the barrel to the ironing or to the bend-elongation working to decrease the thickness of the side wall.
As a method of coating the side-seamless can with an organic film, there has been known a method of laminating a resin film on the metal blank prior to the molding. For example, Japanese Unexamined Patent Publication (Kokai) No. 172556/1994 discloses laminating a polyester film having a limiting viscosity [xcex7] of not smaller than 0.75 on a metal.
It is an accepted practice to blend the polyester with an antioxidizing agent or a reforming component in order to improve the heat resistance and the shock resistance of the polyester used as a resin coating. For example, Japanese Unexamined Patent Publication (Kokai) No. 138387/1995 discloses a polyester film for lamination on a metal, comprising a polyester composition which contains an antioxidizing agent in an amount of from 0.01 to 5% by weight, and Japanese Unexamined Patent Publication (Kokai) No. 207039/1995 discloses a polyester film for lamination on a metal sheet having a melting point of from 120 to 260xc2x0 C., and containing from 0.01% to 1% by weight of a diethylene glycol component and from 0.001 to 1% by weight of an antioxidizing agent.
The side-seamless can is produced by subjecting a resin-coated metal sheet which is obtained by coating in advance, a metal blank with an organic film, to the draw working or to the bend-elongation working. However, the organic coating on the inner surface is likely to be damaged by a tool in the step of working. In the portions where the coating is damaged, the metal is exposed actually or latently, and elutes out or corrodes from these portions. In producing the seamless cans, there takes place a plastic flow in which the size increases in the direction of height of the can and the size contracts in the circumferential direction of the can. When the plastic flow takes place, the adhering force decreases between the surface of the metal and the organic coating and, besides, the adhering force between the two decreases with the passage of time due to residual strain in the organic coating. Such a tendency becomes conspicuous particularly when the content is packaged while it is hot or when the canned content is heat-sterilized at a low temperature or at a high temperature.
Further, the dent resistance is a real shock resistance that is required for the canned products. This is such a property that the adhesiveness and coverage of the coating are still completely maintained even when the canned product is dented as represented by a mark of hit like when the canned product is fallen or when the canned products come into collision with each other. That is, when the coating is peeled or when the coating develops pinholes or cracks in the denting test, the leakage occurs from these portions due to the elution of metal or pitting, and the content is no longer preserved. In general, the polyester which exhibits excellent resistance against the content, lacks the property of absorbing or relaxing the shock at the time of denting test, and imparting this property becomes an important assignment.
Further, the cans for packaging the content are usually printed on the outer peripheral surfaces thereof, and the polyester film is affected by the heat of firing the printing ink. In practically producing the cans, further, the cans are often heated in order to stabilize the resin coating by removing the strain therefrom. The effect of this heating upon the polyester is not negligible. The polyester tends to be thermally deteriorated, i.e., loses the molecular weight by heating accompanied by a decrease in the dent resistance, in the adhesiveness to the metal substrate, in the coating property, and in the workability at the time of necking and wrap-seam working.
In order to improve the dent resistance after thermally deteriorated at such high temperatures, Japanese Unexamined Patent Publication (Kokai) No. 19183/1998 discloses a resin layer applied onto a metal substrate, i.e., discloses a laminate of a polyester or a polyester composition comprising:
a polyester segment derived from (I) a polyethylene terephthalate segment and (II) a butylene glycol and an aromatic dibasic acid;
a polyester segment derived from (III) a butylene glycol and an aliphatic dibasic acid; and
at least one non-sulfur antioxidizing agent having a molecular weight of not smaller than 400 in an amount of from 0.01 to 1.5 parts by weight per 100 parts by weight of the polyester or the polyester composition.
The can comprising the above-mentioned laminate exhibits markedly improved dent resistance after put to the thermal hysteresis at a high temperature, but is not still capable of offering satisfactory corrosion resistance or shock resistance when placed under high temperature and humidity conditions such as during the retort-sterilization or when put to the hot vendor.
Further, Japanese Unexamined Patent Publication (Kokai) No. 195617/1995 discloses a resin-coated metal sheet having, on both surfaces or on one surface of a metal sheet, a film of a resin composition comprising 1 to 25 parts by weight of an ionomer resin, and 75 to 95 parts by weight of a saturated polyester resin which is a compound derived from a dicarboxylic acid and a hydroxy compound and in which, when the dicarboxylic acid component is 100 mol %, the dicarboxylic acid component comprises 50 to 95 mol % of a terephthalic acid and 50 to 5 mol % of an isophthalic acid and/or an orthophthalic acid, and the dihydroxy component is chiefly an ethylene glycol. This resin-coated metal sheet exhibits excellent flavor-retaining property yet exhibiting excellent shock resistance and adhesiveness.
According to the above prior art, however, a particular polyester resin must be prepared. Since the isophthalic acid is fairly expensive, therefore, it is desired to obtain a high degree of shock resistance and adhesiveness as well as flavor-retaining property even when a cheaply available general-purpose polyester resin is used.
If the general-purpose polyester resin is blended with the ionomer resin, further, there occurs a new technical assignment in that lumps are formed due to coagulation of the ionomer resin.
It has further been desired to improve the adhesiveness of the resin coating layer onto the metal substrate and to improve the workability to cope with the production of cans at high speeds. It is further an important technical assignment to improve the resistance against high temperatures and humidity to cope with the retort-sterilization and the subsequent aging even when the content is acidic and is strongly corrosive, to improve the shock resistance after the retort-sterilization, and to improve the corrosion resistance after the retort-sterilization or after having received the shocks.
It is an object of the present invention to provide a resin-coated metal sheet having corrosion resistance, shock resistance (dent resistance) and resistance against high temperature and humidity, which is capable of withstanding the retort-sterilization and the aging in a hot vendor after the retort-sterilization.
It is another object of the present invention to provide a resin-coated metal sheet featuring improved film-forming property, excellent workability, and satisfying a flavor-retaining property required for the cans, by using a general-purpose polyester resin, exhibiting adhesiveness even without using the primer, and making it possible to decrease the cost of production.
It is a further object of the present invention to provide metal cans and can closures having these properties.
According to the present invention, there is provided a resin-coated metal sheet comprising a metal substrate and a thermoplastic resin layer formed on the surface of the metal substrate, wherein the thermoplastic resin layer comprises a polyester consisting chiefly of a polyethylene terephthalate and an ethylene polymer, and contains a tocopherol or a derivative thereof in an amount of from 0.05 to 3% by weight.
In the resin-coated metal sheet of the present invention, it is desired that:
1. The polyester and the ethylene polymer are contained at a weight ratio of from 95:5 to 50:50;
2. The ethylene polymer contains an ionomer;
3. The coated layer has a melt viscosity of from 2000 to 10,000 centipoises at a temperature of 260xc2x0 C. and at a shearing rate of 122 sect, the thermoplastic polyester in the coated layer has an inherent viscosity (IV) in a range of from 0.6 to 1.5, and the ionomer resin in the coated layer is existing as a dispersion phase having an average particle diameter of not larger than 5 xcexcm;
4. The ionomer resin in the coated layer contains zinc as a metal seed; and
5. The coated layer is blended with a novolak resin of a bifunctional phenol.
According to the present invention, further, there is provided a metal can which is so formed that the resin-coated metal sheet becomes the inner surface of the can.
According to the present invention, further, there is provided a metal closure which is so formed that the resin-coated metal sheet is on the inner surface side of the can.